+ correctly copy bytes from the goleveldb store

- this is part of a recent bleve KVStore API change.

    See the following two google group threads for more details:

    * [help adding goleveldb as an alternative Key/Value store for bleve][1]
    * [bleve search performance improvement][2]

    [1]: https://groups.google.com/forum/#!topic/bleve/aHZ8gmihLiY
    [2]: https://groups.google.com/forum/#!topic/bleve/aTyqsSnbhik

index/store/goleveldb/reader.go

@@ -27,6 +27,10 @@ func newReader(store *Store) (*Reader, error) {
 	}, nil
 }
 
+func (r *Reader) BytesSafeAfterClose() bool {
+	return true
+}
+
 func (r *Reader) Get(key []byte) ([]byte, error) {
 	return r.store.getWithSnapshot(key, r.snapshot)
 }

index/store/goleveldb/writer.go

@@ -24,6 +24,10 @@ func newWriter(store *Store) (*Writer, error) {
 	}, nil
 }
 
+func (w *Writer) BytesSafeAfterClose() bool {
+	return false
+}
+
 func (w *Writer) Set(key, val []byte) error {
 	return w.store.setlocked(key, val)
 }